With ZaK's app, you can quickly add more than one guest to your reservations! You need to click on the PLANNER section at the bottom and click on top of the reservation, as in the following example:
Once inside, you'll find an arrow next to the room box:
By clicking on the small arrow, you will see the guest currently registered in that room. You will be able to delete them by clicking on the bin icon or add more guests by clicking on the "Add Guest" item:
☞ Docs: ZaK app - Editing a reservation